“…For simplicity in discussion, only out of plane surface displacement is considered here (for a more detailed analysis, please see ref. 9). Assuming the displacements at (x, y) and (x-x, y-zy) induced by stress are d(x, y) and d(x-x, y-Ay), then, the speckle image intensity can be written as where I (x, y) = a(x, y)+b(x,y)cos((x,y)+ (x, y)) Ø(x, y) =(d(x, y)-d(x-x, y _iy)).4ii/A where the beat fringe pattern sin(A4(x, y)/2) reflects the distribution of surface displacement gradients.…”